Multi-trakcing on Startup?

Hey guys. I have some sweet BGs I made in Gmod and Im wondering if its possible to make a BG or more than one BG with multipule soundtracks? Sorta like the Team Fortress 2 startup with the multipule BGs and Multitracks for it. If a script is needed for it, I can get on that. So if someone can answer my question, great and thanks.